The Public Relations Consultants Association, the Association of Professional Political Consultants, and the Chartered Institute of Public Relations, which have campaigned for a universal register, are united in questioning the breadth of the government's definition of lobbying.

The register would only apply to the activity of lobbying ministers and permanent secretaries on behalf of a third party.

“The definition is weak and so tight as to be almost self-defeating,” said Phil Morgan, the CIPR's director of policy. “How many consultant lobbyists have actually written to or communicated with a minister in their own right?”

“The chances are [that] there are quite senior lobbyists who by a tight reading of that definition might not be included,” he added. “Have they defined a completely new type of lobbying here? Is it going to increase transparency? Probably not.”

PRCA director general Francis Ingham expressed doubts as to whether all 55 agencies on the association's public affairs register would fall in the scope of the statutory register.

“There will be a handful of public affairs practitioners who work in agencies and meet with ministers or permanent secretaries,” he said. “Therefore, the costs of the regulator will be incredibly onerous for the very small number of people who are expected to pay.”

The UK government estimates the cost of the registrar at £500,000 in the first start-up year and £200,000 in subsequent years (in 2013-2014 price terms) and anticipates that these costs will be recovered by a subscription fee from those on the register.

The Cabinet Office's impact assessment for the register assumes there are 1,000 consultant lobbyists in the UK, citing “international comparisons” as well as the UK estimate that the statutory register would cover about 1,000 lobbyists. It further estimates there are 720 lobbying firms.

Any business with annual turnover below the value added tax registration threshold of £79,000 would be exempt from paying the fee.

The impact assessment for the register includes a “central estimate” that the likely annual fee for those firms who have to pay it will be £650 and a “high estimate” of £994.

Ingham added the definition was “full of holes,” saying that people could get round it by meeting with special advisers, backbenchers, or the opposition.

“There's going to be a huge debate over the definition,” said Iain Anderson, deputy chairman of the APPC, who echoed the concerns over the outcome being less transparent.

Anderson said he is also worried about the cost of the regulation, saying it could be “significant” for a lot of small consultancies. Unlike Ingham, he believes all of the APPC's 79 member agencies would fall within the register's scope.

He also drew attention to a difference with the APPC's own voluntary register, which encompasses all agency employees involved in lobbying.

“It looks like the statutory register only applies to the principals of a consultancy rather than employees,” he said, pointing to the bill's definition of a “consultant lobbyist.”

The definition in the bill reads: “A person who carries on the business of consultant lobbying, except that it does not include an individual who does so only as an employee of a person who is a consultant lobbyist.”